Patent GB-208038-A (GB)
Title: Improvements in signalling systems
IPC: H04M3/40
Filed: 1923-01-16
Abstract: 208,038. Western Electric Co., Ltd., (Western Electric Co., Inc.). Jan. 16, 1923. Manual exchange systems are provided with means whereby ringing current is transmitted asa current of a frequency within the voice range interrupted at a lower frequency. A signal is sent from station A, Fig. 1, to station B, by inserting plug 10<1> and pressing key K<1> to connect a ringing current generator 5' to line. This current energizes relay 60<1> which energizes relay 20<1>. Relay 20<1> connects the line through the armature of relay 21<1> to a source of speech-frequency current O<1> and connects a source S<1> of low-frequency current (16 to 20 cycle) to relay 21<1>. Relay 21<1> therefore causes the voice-frequency current sent to the line to be interrupted in accordance with the frequency of source S<1>. The interrupted current passes through speech amplifiers R', R to a tuned circuit 22, 24, 25, 26, and is rectified by a valve V, the anode circuit of which is tuned to the frequency of the interruptions. The rectified current operates an alternating current relay 29 which causes condenser 32 to be charged alternately in opposite directions and to discharge through relay 33 which operates and short-circuits relay 44. Relay 44 energizes a slowto operate relay 45 which connects a source 46 of ringing current to line to operate the indicator 14. Relay 33 is designed so as not to operate continuously except in response to currents of the proper frequency. Signalling from station B to station A is effected in the same way. Any suitable signalling device may be provided in the cord circuit to admit of signalling when the plug 10 at the receiving station is in its jack. In an alternative arrangement, Fig. 2, ringing frequency currents pass from line L at the trans. mitting station through a filter F<1> and relay 111 to a " polar relay " 114. Relay 111 energizes relay 128 which switches over transformer 119 from the voice-frequency receiving to the voicefrequency transmitting apparatus. "Polar relay" 114, in response to the ringing frequency, alternately charges and discharges condenser 116 in an oscillating circuit including, during discharge, inductance 115 and transformer 119, and during charge condenser 155 also. This circuit is tuned to about 800 so that for each charge and discharge a damped train of this frequency is sent to line. These trains pass at the receiving station from line L through transformer 119 and series-tuned circuit 132 of slightly lower frequency than 800 to an amplifier detector 134 the plate circuit of which is tuned to the wave-train frequency. The rectified currents operate a " polar relay " 137 which alternately charges condenser 138 and causes it to discharge through relay 140. This relay, which is designed to operate only in response to currents of the proper frequency, releases normally-energized relay 146 after a time interval depending on the dimensions of condenser 157 and the right-hand resistance 150. Relay 146 energizes relay 109 which connects the line to a source 148 of low-frequency current to operate the switchboard apparatus. At the close of the ring, relay 137 stops vibrating and relay 146 re-energizes, but with a time-lag depending on the dimensions of condenser 157 and its discharge path which now includes both resistances 150 in parallel.